We will never be able to prevent a small group of misfits from planning some terrible act of terror. No matter how far-seeing and competent our intelligence and law-enforcement officials, people will always be able to slip through the cracks in a large, open and diverse country. The real test of American leadership is not whether we can make 100 percent sure we prevent the attack, but rather how we respond to it. Stephen Flynn, a homeland-security expert at the Council on Foreign Relations, argues that our goal should be resilience—how quickly can we bounce back from a disruption? In the materials sciences, he points out, resilience is the ability of a material to recover its original shape after a deformation. If one day bombs do go off, we must ensure that they cause as little disruption—economic, social, political—as possible. This would deprive the terrorist of his main objective. If we are not terrorized, then in a crucial sense we have defeated terrorism.

The atmosphere of fear and panic we are currently engendering is likely to produce the opposite effect. Were there to be another attack, politicians would fulfill their pledges to strike back, against someone. A retaliatory strike would be appropriate and important—if you could hit the right targets. But what if the culprits were based in Hamburg or Madrid or Trenton? It is far more likely that a future attack will come from countries that are unknowingly and involuntarily sheltering terrorists. Are we going to bomb Britain and Spain because they housed terror cells?